March 2017 was warm and wet, with a major storm to finish out the month. Without that storm, march would actually have been slightly drier than average. But with 2.86 inches, it turned out to be the second wettest march at the Boise airport, and the seventh wettest march going back to 1878 in the Boise area. The 1.40 inches on the 30th tied January 16, 1896 as the tenth wettest day in the Boise area. At the airport it was the seventh wettest day.
There were showers each day from the 3rd through the 11th, and there was measurable precipitation on nearly half the days from the 12th through the 31st.
The average temperature of 48.9 degrees tied 1992 as the second warmest march at the Boise airport, and the fifth warmest march going back to 1878 in the Boise area.
There were no record high temperatures, but the lows of 52 on the 18th and 48 on the 20th broke the previous daily records for warmest lows. The 49 on the 19th tied the record high low for that date. There were no freezing temperatures from the 8th through the 27th.
During the first 28 days of the month there were no significant storms. Predominantly westerly or southwesterly flow aloft and eastward migrating high pressure ridges kept temperatures mild. Minor weather systems moving inland from the Pacific provided light precipitation.
On the morning of the 29th, an ordinary looking upper level trough off the British Columbia coast was approaching the northwest U.S. By evening it had noticeably deepened as it neared the Washington coast. A third of an inch of rain fell at the airport that evening, way out ahead of the main storm.
The rain stopped before midnight, only to resume just before 7 am MDT on the 30th as the cold front approached. As the front passed Boise around 8:30 am, the rain became heavy, and west winds increased to around 25 mph. The rain changed to snow just before 10 am, but only a trace fell before the snow ended shortly after 11 am.
Just over one inch of precipitation fell in 6 hours on the morning of the 30th. Interestingly, the amount of moisture available for precipitation, measured by radiosonde during the two hours before the rain started, was a mere half inch. So how could nearly three times that amount fall on Boise?
As the upper level trough rapidly intensified directly over southwest Idaho, more moisture was pulled in from outlying areas and lifted over the treasure valley. At the same time, the strong cold front provided additional lift to turn that moisture into rain and snow. Also, a closed circulation developed aloft, slowing the storm movement. This allowed more time for precipitation to accumulate.
As the system strengthened at upper levels, surface low pressure to our east deepened, causing northwest winds to increase through the afternoon. A peak gust of 52 mph was measured at the airport at 4:06 pm.
